Method for concentrating dilute acidic solutions

ABSTRACT

Dilute aqueous acidic solutions are concentrated by direct contact with a hot gas stream in a venturi contactor, under specific conditions of the ratio of total solution flow rate to hot gas flow rate, and hot gas flow velocity at the throat section of the venturi passage, so that the acidic liquid solution is concentrated by evaporation of water vapor into the hot gas stream without the formation of an acid mist or other contaminants in the gas stream.

United States Patent 1191 Shah et a1.

[451 Feb. 5, 1974 METHOD FOR CONCENTRATING DILUTE ACIDIC SOLUTIONS [75]Inventors: Indravadan S. Shah, Forest Hills,

N.Y.; John B. Rinckhoff, Lakeland, Fla.

[73] Assignees Chemical Construction Corporation,

New York, N.Y.

[22] Filed: June 18, 1971 [21] Appl. No.: 154,366

[56] References Cited UNITED STATES PATENTS Farin 159/4 A X |2 FURNACESTREAM l5 ORGANIC 3 STRH" REACTQIR 8 l3 2,818,917 1/1958 Vincent 159/483,057,700 10/1962 Gross 23;159/383;13 C UX 3,211,538 10/1965 Gross et a1v 159/4 A X 3,275,062 9/1966 Williams 159/4 A 3,439,724 4/1969 Mason159/13 C 3,574,051 4/1971 Shah 159/47 WL 3,349,546 10/1967 Rogers 55/227Primary Examiner-Jack Sofer Attorney, Agent, or Firm-.1. L. ChabotyABSTRACT Dilute aqueous acidic solutions are concentrated by directcontact with a hot gas stream in a venturi contactor, under specificconditions of the ratio of total solution flow rate to hot gas flowrate, and hot gas 19W. vq wa the h atseq of the. vp rl ar sage, so thatthe acidic liquid saluti n is concentrated by evaporation of water vaporinto the hot gas stream without the formation of an acid mist or othercontaminants in the gas stream.

7 Claims, 1 Drawing Figure cnuos PROD.

RECYCLE conc. SOL.

SULPHURIC AClD SEPARATOR couc. ACID PATENTEU 5 74 FURNACE ORGANIC STREAM3 REACT 'RECYCLE /CONC. SOL.

SULPHURIC /AC|D SEPARATORQ VI 4T m w MS 2 M VA HF mG D T A W 27 TAIL)GAS FURNACE JOHN B. R I NCKHOFF INDRAVADAN S. SHAH INVENTORS. BY?

AGEN'T METHOD FOR CONCENTRATING DILUTE ACIDIC SOLUTIONS BACKGROUND OFTHE INVENTION 1. Field of the Invention The invention relates to theevaporative concentration of dilute aqueous acidic solutions, such asspent sulfuric acid typically derived from petroleum refining or themanufacture of methyl methacrylate or caprolactam. These spent acidsmust be concentrated, prior to combustion in furnaces which generate asulfur dioxide-rich gas stream for usage in sulfuric acid manufacture.Other dilute aqueous acidic solutions which may be concentrated by themethod of the present invention include pickle liquors derived from thetreatment of metallic surfaces with acids, acid mine waste waters,aqueous solutions of organic acids, and the like.

2. Description of the Prior Art The evaporative concentration of asludge acid prior to high temperature decomposition is described in US.Pat. No. 2,406,930. The general concept of evaporation of liquidsolutions in Venturi-type contactors is generally disclosed in US. Pat.Nos. 2,1 18,803; 2,937,141; 3,211,538; 3,212,235; 3,275,062; 3,284,064;3,349,546 and 3,439,724, and in the publication Chemical Engineering,issue for Aug. 26, 1968, pages 54 and 56. In practice, sludge acids andthe like have generally been commercially concentrated in drum-likeconcentrators or the like.

SUMMARY OF THE INVENTION In the present invention, dilute aqueous acidicsolutions are concentrated in an improved manner, by means of a methodin which the solution is contacted with a hot drying gas within avertically oriented venturi passage or a contactor of the venturiconfiguration. The dilute aqueous acidic solution flows downwards on theinner surface of the converging passage and the hot gas stream,generated or produced at an initial temperature above 200C, is passeddownwards through the venturi passage. The ratio of total solution flowrate to hot gas flow rate is maintained in the range of 1 liter peractual cubic meter to liters per actual cubic meter, and the diluteliquid solution is projected into the high velocity hot gas at thethroat section of the venturi passage, so that water is evaporated fromthe aqueous solution into the gas stream. A gas velocity in the range of10 meters/second to 60 meters/second is maintained within the throatsection of the venturi passage. The resulting concentrated liquidsolution is separated from the moisture-laden cooled gas stream which isnow at a reduced temperature in the range of 100C to 150C.

The principal advantage of the invention is that operation within theranges and limitations of the method as enumerated supra serves toeffectively concentrate the acidic liquid solution without the formationof an acid mist or fog, and without the generation of acid decompositionproducts in the cooled exit gas stream. Another advantage is that themethod is low in operating cost, and the equipment is of low capitalcost.

It is an object of the present invention to provide an improved methodfor the concentration of dilute aqueous acidic solutions.

Another object is to provide a method for evaporative concentration ofdilute aqueous acidic solutions by contact with a hot drying gas.

A further object is to concentrate dilute aqueous acidic solutions byevaporative contact with a hot drying gas, without the formation of anacid mist.

An additional object is to provide an improved method for evaporativeconcentration of a dilute aqueous acidic solution by contact with a hotdrying gas in a venturi contactor.

These and other objects and advantages of the present invention willbecome evident from the description which follows.

DESCRIPTION OF THE DRAWING AND PREFERRED EMBODIMENTS Referring now tothe drawing, a flowsheet of a preferred embodiment of the invention isillustrated, in which a dilute aqueous acidic solution derived from themanufacture of methyl methacrylate and principally containing dissolvedammonium bisulfate and sulfuric acid is concentrated from an initialwater content of about 41 percent by weight to a final water content ofabout 20 percent by weight, prior to sulfuric acid recovery from theacidic residue.

Organics stream 1 and recycled concentrated sulfuric acid stream 2 arepassed into reactor 3, from which product crude methyl methacrylate iswithdrawn via stream 4. A dilute aqueous acidic solution stream 5 isalso withdrawn from unit 3, and stream 5 typically contains in the rangeof 30 percent to 60 percent water content by weight, together withdissolved ammonium bisulfate, sulfuric acid and organics. Stream 5 ispreferably combined with recycle concentrated acidic solution stream 6,and the combined solution stream 7 is passed via one or a plurality offeed nozzles 8 onto the upper liquid distribution shelf 9 of the venturiconcentrator. The nozzles 8 are preferably disposed substantiallytangential to the circular shelf 9, so that the liquid flows from theshelf 9 and downwards on the inner surface of the invertedfrusto-conical baffle 10 with a whirling circular motion. Baffle 10forms the converging approach or inlet section of the venturicontactorconcentrator, which is also defined by the cylindrical throatbaffle 11 which depends from baffle 10 and defines the throat of theventuri contactor, in which the hot drying gas is accelerated to highvelocity for contact with dispersed liquid, as will appear infra.

A hot drying gas generated or produced and derived from any suitablesource is passed downwards through the venturi passage defined bybaffles l0 and 11. In this preferred embodiment of the invention, thehot drying gas is formed by burning fluid hydrocarbon stream 12 withcombustion air stream 13 in furnace 14. Stream 12 may consist of anysuitable or available hydrocarbon, such as methane, propane, butane,crude oil, fuel oil, petroleum refinery residual oil, or the like. Thehot drying gas stream 15 emitted by furnace 14 is at a temperaturegenerally above 200C and preferably in the range of 200C to 400C, andstream 15 is now passed into the venturi concentrator and downwardswithin baffles 10 and 11.

The ratio of the flow rates of streams 7 and 15 is maintained in therange of between 1 liter per actual cubic meter and 10 liters per actualcubic meter, in order to attain the improved results of the presentinvention. In addition, in order to attain these improved results,stream 15 is accelerated to high velocity in flowing downwards throughthe converging passage defined by baffle 10, so that the gas velocitywithin the throat section defined by baffle 1 l is in the range ofmeters per second to 60 meters per second.

The downflowing liquid solution on the inner surface of baffle 10 flowsdownwards and is projected into the high velocity gas stream withinthroat 11, so that the liquid is dispersed into small droplets and rapidgasliquid equilibrium is attained with concomitant rapid evaporation ofwater from the liquid into the gas phase. in addition, due to themaintenance of operating parameters as discussed supra, formation of anacid mist or fog is prevented and the gas phase does not contain anyappreciable proportion of acid or acid decomposition products. I

The resultant gas-liquid droplets phase formed in throat section 11 ispassed downwards through the lower frusto-conical baffle 16 whichdepends from baffle l1 and is preferably provided as a diverging sectionof the venturi passage in order to conserve gas pressure drop. Thegas-liquid mixture next flows into entrainment separator 17, which isany suitable means or device for separating liquid droplets from a gasstream. Thus in suitable instances unit 17 may be baffled or cyclonic orthe like. The separated gas phase is removed from section 17 via duct18, which discharges the cooled and moisture-laden gas stream 19 to astack or the like for atmospheric dispersal, which may be carried outwithout danger of air pollution, due to the absence of acid mist or fog,or acid decomposition products in the cooled gas stream 19 which is nowtypically at a temperature in the range of 70C to 150C.

A pool or layer of concentrated acidic solution collects in the bottomof unit 17, and a stream of concentrated solution is removed from unit17 via nozzle 20 as stream 21, which now typically contains in the rangeof 10 percent to percent water content. Stream 21 is now preferablydivided into stream 6, which is recycled as described supra, and stream22, which is suitable for combustion to generate a sulfur dioxide-richgas stream. Stream 22 is passedinto combustion furnace 23 together withcombustion air stream 24, and the resultingsuflur dioxide-rich gasgenerated in furnace 23 is preferably passed via stream 25 intosulfuricacid production facility 26, in which the gas stream is ric acidsolutions such as dilute aqueous solutions de rived from the manufactureof methyl methacrylate or caprolactam, or sludge acids derived frompetroleum refining or the like. In some instances the concentratedsolution recycle stream 6 may be omitted, in which case the totalsolution stream 7 would consist solely of dilute aqueous acid solutionsuch as stream 5. Stream 15 may alternatively be a flue gas or the like,derived from a steam boiler or other suitable source. The baffle 16 maybe omitted in suitable instances. or replaced by a cylindricalconnecting member which would extend to unit 17.

An example of pilot plant testing of the method of the mssestinxs t sw wt b 95902;

Example The method of the invention was tested in a series of runs in anacid concentrator pilot plant. The feed acid was a residual spent acidfrom methyl methacrylate manufacture, which had the followingcomposition, by weight:

Water 41% Sulfuric Acid 6.5% Ammonium Acid Sulfate 50.0% Organics 2.5%

All runs utilized a 425C feed gas obtained by burning natural gas in afurnace. The acid feed rate was set at a maximum value that would give20 percent by weight water in the product acid. The acid recycle rateand venturi throat diameter was varied to determine their effect on theprocess conditions. An acid residue containing 20 percent water wasproduced in the venturi concentrator. in general, the concentratoroff-gas contained less than 5 mg./cu.ft. sulfuric acid mist which wasreduced to less than 2 mg./cu.ft. at a pressure drop greater than 6inches of water. The sulfur dioxide content was nil and sulfur trioxideless than 5 ppm.

The venturi concentrator was a standard venturi scrubber, 30 inches indiameter and 19 ft. high. Following is a summary of test results.

Summary of Test Results Run No. 1 2 3 4 5 6 7 8 Length of Run, hrs 2 3 77 7 7 7 7 Initial Gas Temp. (C).. 442 430 435 433 431 431 430 430 FinalGasTemp. (C) 135 122 120 119 116 119 Recycle Acid Temp. (C) 99 95 97 9796 96 97 96 Recycle Acid Flow Rate (gpm)... 36 23.2 23.5 23.5 35.0 35.034.8 34.8 Feed Acid Flow Rate (gpm) 2.5 2.07 2.05 2.05 2.2 2.2 2.5 2.45Final Gas Analysis:

Mist, mg.lft.-...... 5.0 2.5 6.21 5.04 5.54 0.0 3.36 3.68 Entertainment,mg./ft 5.8 9.14 22.6 31.8 41.0 0.0 42.6 55.9 502. ppm 0 0 0 0 0 14 0 0S03, Ppm.... 53 61 84 84 34 48 30 30 N0 ppm 0 0 27 dried, the sulfurdioxide content of the dried stream 25 is catalytically oxidized tosulfur trioxide and the sulfur trioxide is absorbed in concentratedsulfuric acid to form further sulfuric acid, which is withdrawn fromunit 26 via stream 2 and recycled as described supra. A tail gas stream27 substantially devoid of sulfur oxides is also discharged from unit26.

Numerous alternatives within the scope of the present invention willoccur to those skilled in the art. The present method is especiallyapplicable to spent sulfu- This test program proved the feasibility ofthe method of the present invention, for reducing the water content ofacid residues from 41 percent to 20 percent by weight. It also indicatedthat a wider range could be handled. It was found that the sulfuric acidmist content is less than 5 mg./cu.ft. and could be reduced to less than2 mg./cu.ft. if a pressure drop greater than 6 in. of water across theconcentrator is maintained.

The pressure drop was varied by changing the throat Size n l q id, t9 aa i nqiqa sth e yc e rate was increased, the pressure drop increasedfrom 3.9 to 6.3 in. of water, and the approach temperature of gas toliquid decreased from 35 to 20C. Therefore, not only did the increasedpressure drop reduce the acid mist level in the exit gas but itsignificantly improved the heat transfer efficiency. This allowed apercent increase in feed acid flow rate. The operating parameters andranges enumerated supra are directly related to gas pressure drop.

We claim:

1. A method for concentrating a dilute aqueous spent sulfuric acidsolution containing in the range of 30 percent to 60 percent watercontent to a concentrated sulfuric acid solution containing in the rangeof 10 percent to 25 percent water content which comprises passing a hotgas stream downwards through a vertically oriented converging venturipassage provided with a lower restricted throat section, said hot gasstream being at an initial temperature above 200C, flowing a combinedaqueous acidic solution downwards on the inner surface of saidconverging passage at a ratio of total solution flow rate to hot gasflow rate in the range of 1 liter per actual cubic meter to 10 litersper actual cubic meter respectively, said combined aqueous acidicsolution being formed by adding a concentrated sulfuric acid solutionportion to said dilute aqueous spent sulfuric acid solution, wherebysaid combined aqueous acidic solution is projected into the highvelocity hot gas at the throat section of said venturi passage and wateris evaporated from said combined aqueous solution into the gas stream,said hot gas flow rate providing a gas velocity at the throat section ofsaid venturi passage in the range of 10 meters/second to 60 meters/-second, whereby the formation of an acid mist is prevented, removing agas-liquid mixture from below said venturi passage, said gas-liquidmixture consisting of droplets of concentrated sulfuric acid solutiondispersed in said gas stream, separating a stream of concentratedsulfuric acid solution containing in the range of 10 percent to 25percent water content from the gasliquid mixture, the remaininggas-liquid mixture comprising a gas-vapor mixture containing water vaporderived from said solution and being at a reduced temperature in therange of C to C is wasted to atmosphere and dividing said concentratedsulfuric acid solution into a first portion and a second portion, saidfirst portion being recycled as said concentrated sulfuric acid solutionportion of said combined aqueous acidic solution, and said secondportion being withdrawn as product concentrated sulfuric acid solutioncontaining in the range of 10 percent to 25 percent water content andthe balance essentially sulphuric acid.

2. The method of claim 1, in which said spent sulfuric acid solutioncontains ammonium bisulfate and is derived from the manufacture ofmethyl methacrylate.

3. The method of claim 1, in which said spent sulfuric acid solution isderived from the manufacture of caprolactam.

4. The method of claim 1, in which said spent sulfuric acid solution isa sludge acid derived from petroleum refining.

5. The method of claim 1, in which said hot gas stream is a flue gasderived from a fuel combustion process.

6. The method of claim 1, in which said hot gas stream is produced at aninitial temperature in the range of 200C to 400C by burning a fluidhydrocarbon in air.

7. The method of claim 6, in which said fluid hydrocarbon is selectedfrom the group consisting of methane, propane, butane, crude oil, fueloil and residual oil derived from petroleum refining.

1. A method for concentrating a dilute aqueous spent sulfuric acidsolution containing in the range of 30 percent to 60 percent watercontent to a concentrated sulfuric acid solution containing in the rangeof 10 percent to 25 percent water content which comprises passing a hotgas stream downwards through a vertically oriented converging venturipassage provided with a lower restricted throat section, said hot gasstream being at an initial temperature above 200*C, flowing a combinedaqueous acidic solution downwards on the inner surface of saidconverging passage at a ratio of total solution flow rate to hot gasflow rate in the range of 1 liter per actual cubic meter to 10 litersper actual cubic meter respectively, said combined aqueous acidicsolution being formed by adding a concentrated sulfuric acid solutionportion to said dilute aqueous spent sulfuric acid solution, wherebysaid combined aqueous acidic solution is projected into the highvelocity hot gas at the throat section of said venturi passage and wateris evaporated from said combined aqueous solution into the gas stream,said hot gas flow rate providing a gas velocity at the throat section ofsaid venturi passage in the range of 10 meters/second to 60meters/second, whereby the formation of an acid mist is prevented,removing a gas-liquid mixture from below said venturi passage, saidgasliquid mixture consisting of droplets of concentrated sulfuric acidsolution dispersed in said gas stream, separating a stream ofconcentrated sulfuric acid solution containing in the range of 10percent to 25 percent water content from the gas-liquid mixture, theremaining gas-liquid mixture comprising a gas-vapor mixture containingwater vapor derived from said solution and being at a reducedtemperature in the range of 70*C to 150*C is wasted to atmosphere, anddividing said concentrated sulfuric acid solution into a first portionand a second portion, said first portion being recycled as saidconcentrated sulfuric acid solutioN portion of said combined aqueousacidic solution, and said second portion being withdrawn as productconcentrated sulfuric acid solution containing in the range of 10percent to 25 percent water content and the balance essentiallysulphuric acid.
 2. The method of claim 1, in which said spent sulfuricacid solution contains ammonium bisulfate and is derived from themanufacture of methyl methacrylate.
 3. The method of claim 1, in whichsaid spent sulfuric acid solution is derived from the manufacture ofcaprolactam.
 4. The method of claim 1, in which said spent sulfuric acidsolution is a sludge acid derived from petroleum refining.
 5. The methodof claim 1, in which said hot gas stream is a flue gas derived from afuel combustion process.
 6. The method of claim 1, in which said hot gasstream is produced at an initial temperature in the range of 200*C to400*C by burning a fluid hydrocarbon in air.
 7. The method of claim 6,in which said fluid hydrocarbon is selected from the group consisting ofmethane, propane, butane, crude oil, fuel oil and residual oil derivedfrom petroleum refining.